1. Technical Field
The present invention relates to an image compressing method, an image compressing apparatus and an image forming apparatus in which a color image is compressed.
2. Description of Related Art
Digital image processing systems have been remarkably developed in recent years, and establishment of digital image processing techniques have been proceeded. In the field of, for example, copying machines and multi-function printers (MFPs) employing an electrophotographic method or an inkjet method, a document of a manuscript is read with a scanner to be stored as a document file of electronic data and the thus stored document file is managed. Furthermore, a document file is compressed to be submitted with an e-mail.
Since an image read with a scanner (hereinafter referred to as a scanned image) has a large file size in general, it is indispensable to compress a scanned image for storage or transmission. As one compression technique for compressing such an image in a high compression ratio, an image compression technique based on layer segmentation such as Mixed Raster Content (MRC) has been put to practical use.
In the image compression technique based on the layer segmentation, a foreground mask corresponding to a text and/or a line art is generated from a color image to be compressed, the color image is separated into a foreground layer and a background layer on the basis of the generated foreground mask, and the foreground layer and the background layer are respectively compressed by employing suitable compression techniques, so as to ultimately generate a highly compressed image (see Japanese Patent Application Laid-Open No. 2002-94805). At this point, a foreground layer is a layer of a foreground corresponding to a text and/or a line art and is generally compressed by employing a lossless compression technique such as JBIG (Joint Bilevel Image Group), MMR (Modified Modified Read code) or LZW (Lempel Ziv Welch).
On the other hand, a background layer is a layer of a background corresponding to an image content other than the text and/or line art and is generally compressed by employing a lossy compression technique such as JPEG (Joint Photographic Experts Group). In compression attained by the lossy compression technique, the image quality of a compressed image is more largely degraded than in compression attained by the lossless compression technique. Since the compression ratio is easily controlled in the lossy compression technique, however, priority can be placed on a small file size or high image quality in accordance with the use of a compressed image. On the contrary, since the compression ratio is difficult to control in the lossless compression technique, it is difficult to improve the compression ratio by employing this technique.